The proton symporter MntH serves as the sole high affinity manganese transporter in Brucella strains (2). The extreme attenuation displayed by a B. abortus mntH mutant in experimentally infected mice indicates that Mn plays an exceptionally important role in the virulence of Brucella strains in comparison with other bacterial pathogens that have been examined. Glucose catabolism has recently been shown to be critical for the ability of Brucella strains to persist in alternatively activated macrophages, which in turn is required for the maintenance of chronic infections in experimentally infected mice (51). Brucella strains catabolize glucose exclusively via the pentose phosphate pathway (15), and the activities of two enzymes required for maintaining carbon flow through this pathway, pyruvate kinase (PykM) (20) and ribulose-5-phosphate 3-epimerase (Rpe) (46), are predicted to the particularly susceptible to reduced cellular Mn levels. Consequently, the studies outlined in Specific Aim 1 of this application will test the hypothesis that defective glucose catabolism resulting from reduced PykM and Rpe activities makes a significant contribution to the attenuation exhibited by the B. abortus mntH mutant. Experimental evidence also suggests that Brucella strains have the capacity to increase their MntH-mediated Mn uptake and substitute this metal for iron in cellular proteins as a protective mechanism against oxidative stress, in a similar manner to that recently demonstrated in Escherichia coli (3). The studies described in Specific Aim 2 of this proposal will identify the transcriptional regulator responsible for the H2O2- responsive induction of mntH expression in B. abortus 2308, and determine if this mode of mntH regulation protects this strain from oxidative stress in vitro and is required for its virulence in mice. Results obtained from the proposed studies will begin to explain Mn plays such a critical role in the basic physiology and virulence of Brucella strains. The fact that loss f MntH has such an adverse effect on virulence also makes this transporter an attractive target for the development of improved vaccines and antibiotics to prevent and treat human brucellosis, a major zoonotic disease of worldwide importance.